Alpha Transparency Lightening and Volume Scatter
I deleted my earlier "answers" as they turned out to be mostly useless. The answer seems to be to use Branched Path Tracing in the cycles render settings. This eliminates the effect of the alpha transparency outline of the images as planes showing up when using volume scatter.
Blender HD has an excellent tutorial on all the obscure settings in cycles render, explaining with great examples what each one does. https://www.youtube.com/watch?v=NhOdnOow6DY&amp;t=394s